Video display systems, including video games, often require some type of manipulative control element to form an electrical signal from which movement of a symbol (or other information change) on a display screen can be effected. One form of such control element is a joy stick having two (2) degrees of freedom (forward-reverse and left-right). Resultant motion of the "stick" will operate to close a switch or control a potentiometer to produce a voltage that is ultimately used to position an object (or produce another change in the display).
Also available is a ball operated mechanism, the ball forming the manipulative element. The ball is operably connected to suitable mechanical structure for providing output signals that can be used to position symbols on a video display screen. In one form of this type of mechanism the ball is supported by a motion transmitting mechanism, including a pair of orthogonally oriented shafts. Universal movement of the ball resolves into rotational movement in one or both of the shafts to produce signals indicative of ball movement. Examples of this type of control element are found in U.S. Pat. Nos. 3,013,441 and 3,395,589.
A shaft encoder of one type or another is coupled to the shafts of this latter type of control element to provide an output indicative of shaft rotation--and, therefore, ball movement. One form of shaft encoder uses a disk attached to an end of a shaft that is apertured to modulate a collimated light beam as the shaft rotates about its longitudinal axes. An example of this type of encoder may be found in U.S. Pat. No. 4,271,364. The modulated light beam is received by an appropriate detector which produces a stream of pulses therefrom. The pulse recurrance frequency (PRF) of the stream is indicative of the rate of rotation of the shaft. In order to resolve direction of rotation of the shaft, two phased detectors are used, each being placed proximate the disk to receive the modulated light beams in a predetermined sequence. The stream of pulses produced by one detector will lead or lag (typically by 90 degrees) the stream of pulses produced by the other detector-depending upon the direction of shaft rotation.
However, using such shaft encoding systems is not without certain problems. For example it is often costly in terms of the interrupts, time, and software required to read directly the pulse stream pairs produced for each shaft. Further, the present invention requires only two pin connections for each control element.